1. Field of the Invention
The present invention relates to tandem photovoltaic cells, photovoltaic cell modules comprising the tandem photovoltaic cells, a method of fabricating the tandem photovoltaic cells, and a method of repairing the tandem photovoltaic cells.
2. Description of the Background Art
Tandem photovoltaic cells have been developed for enhancing the photovoltaic conversion efficiency. FIG. 12 is a cross section showing an example of a thin film photovoltaic conversion module that includes conventional tandem photovoltaic conversion cells (refer to JP 2002-261308 A, for example). The thin film photovoltaic conversion module shown in FIG. 12 has a plurality of photovoltaic conversion cells 120. Each photovoltaic conversion cell 120 has a tandem structure that includes a transparent front electrode layer 122, an amorphous photovoltaic conversion layer 123, an intermediate reflective layer 124, a crystalline photovoltaic conversion layer 125, and a back electrode layer 126 in order on a transparent substrate 121. Adjacent photovoltaic conversion cells 120 are separated from one another through separation grooves 127. The transparent front electrode layer 122 of each photovoltaic conversion cell 120 is connected to the back electrode 126 of an adjacent other photovoltaic conversion cell 120. In this way, the plurality of photovoltaic conversion cells 120 are serially connected.
Such tandem photovoltaic conversion cells 120 are capable of absorbing light in a wide range of wavelengths through the amorphous photovoltaic conversion layer 123 and the crystalline photovoltaic conversion layer 125, thus providing high conversion efficiency. In addition, the provision of the intermediate reflective layer 124 allows an increase in the amount of light incident to the amorphous photovoltaic conversion layer 123. This enables an increased amount of power that is generated in the amorphous photovoltaic conversion layer 123.
The present inventors, however, found that such a thin film photovoltaic conversion module with an intermediate reflective layer has following problems occurring during the formation of separation grooves by etching.
FIG. 13 is a diagram for illustrating the problems during the formation of separation grooves. When forming a separation groove 132 in the thin film photovoltaic conversion module shown in FIG. 13 by etching, the intermediate reflective layer 131 and the back electrode 133 are more difficult to etch than the first photovoltaic conversion layer 134 and the second photovoltaic conversion layer 135. This causes side-etching of the first photovoltaic conversion layer 134 and the second photovoltaic conversion layer 135. Consequently, as shown in FIG. 13, the side surfaces of the first photovoltaic conversion layer 134 and the second photovoltaic conversion layer 135 are concave, and the side ends of the intermediate reflective layer 131 and the back electrode 133 remain in the form of girders toward the separation groove 132 side. If the remaining girder-formed portions of the intermediate reflective layer 131 and the back electrode 133 bend to contact a light receptive electrode 136 and the intermediate reflective layer 131, this causes short circuits in the first photovoltaic conversion layer 134 and the second photovoltaic conversion layer 135.
During the fabrication process of a thin film photovoltaic conversion module, a short circuit may also occur between the electrodes when a pinhole develops in a photovoltaic conversion layer of a photovoltaic conversion cell.
For thin film photovoltaic conversion modules comprising single-layer photovoltaic conversion cells, a method for recovering short-circuited portions is suggested. According to the method, when a short circuit occurs between the electrodes of a photovoltaic conversion cell during the fabrication process, a reverse bias voltage is applied between the electrodes for repairing the short-circuited portion (refer to e.g. JP 2000-277775 A). In the specification, this repairing method will be referred to as “reverse bias repair”.
FIG. 14 is a diagram showing the equivalent circuit of a single-layer photovoltaic conversion cell during reverse bias repair. A photovoltaic conversion layer, which is equivalent to a diode, functions as a capacitor C3 when a reverse bias voltage is applied. A short-circuited portion, which is equivalent to a resistance, is represented by R.
When a reverse bias voltage is applied between the electrodes, a current selectively flows in the short-circuited portion R to generate Joule's heat. The Joule's heat causes the metal of the short-circuited portion R to become an insulator through oxidization or causes the metal to fly to pieces, thereby recovering the short-circuited portion R.
The above-described reverse bias repair, however, is not applicable to tandem photovoltaic conversion cells, because of the reasons as described below.
FIG. 15 is a diagram showing the equivalent circuit of a tandem photovoltaic conversion cell with two photovoltaic conversion layers when a reverse bias voltage is applied. The photovoltaic conversion layers in the tandem photovoltaic conversion cell, which are equivalent to diodes, function as capacitors C4, C5, respectively, when a reverse bias voltage is applied. A short-circuited portion, which is equivalent to a resistance, is represented by R.
The presence of the capacitor C4 between one electrode and the short-circuited portion R inhibits a current to flow in the short-circuited portion R when a reverse bias voltage is applied between the electrodes. For this reason, the reverse bias repair is not applicable to tandem photovoltaic conversion cells. It has therefore been difficult to recover short-circuited portions in photovoltaic conversion modules with tandem photovoltaic conversion cells.